1. Field of the Invention
The present invention relates to an in-plane switching (IPS) mode liquid crystal display (LCD) device, and more particularly, to an IPS mode LCD device having improved ambient aperture ratio and image quality and being fabricated by reduced mask processes and a method of fabricating the same.
2. Discussion of the Related Art
The related art liquid crystal display (LCD) device is widely used because of advantages in power consumption and portability and value-added technology. Generally, the LCD device is driven by using optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by application of an electric field across the liquid crystal molecules. As the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmissivity.
Since the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics such as high resolution and display of moving images, the AM-LCD device has been widely used.
The AM-LCD device includes an array substrate, a color filter substrate and a liquid crystal layer interposed therebetween. The array substrate may include a pixel electrode and TFT, and the color filter substrate may include a color filter layer and a common electrode. The AM-LCD device is driven by an electric field between the pixel electrode and the common electrode resulting in excellent properties of transmittance and aperture ratio. However, since the AM-LCD device uses a vertical electric field, the AM-LCD device has a bad viewing angle.
An in-plane switching (IPS) mode LCD device may be used to resolve the above-mentioned limitations. The related art IPS mode LCD device includes a color filter substrate, an array substrate facing the color filter substrate, and a liquid crystal layer interposed therebetween. Both common and pixel electrode for driving the liquid crystal layer are formed on the array substrate.
FIG. 1 is a schematic cross-sectional view of the related art IPS mode LCD device. Referring to FIG. 1, the IPS mode LCD device includes first and second substrates 9 and 10 facing each other and a liquid crystal layer 11 therebetween.
On the second substrate 10, both of a common electrode 17 and a pixel electrode 30 are formed. The liquid crystal molecules of the liquid crystal layer 11 are driven by a horizontal electric field L generated between the common and pixel electrodes 17 and 30.
FIGS. 2A and 2B are cross-sectional views showing turned on/off conditions of the related art IPS mode LCD device. As shown in FIG. 2A, when the voltage is applied to the IPS mode LCD device, liquid crystal molecules 11a above the common electrode 17 and the pixel electrode 30 are unchanged. But, liquid crystal molecules 11b between the common electrode 17 and the pixel electrode 30 are horizontally arranged due to the horizontal electric field “L”. Since the liquid crystal molecules are arranged by the horizontal electric field, the IPS mode LCD device has a characteristic of a wide viewing angle.
FIG. 2B shows a condition when the voltage is not applied to the IPS mode LCD device. Because an electric field is not generated between the common and pixel electrodes 17 and 30, the arrangement of liquid crystal molecules 11 is not changed.
FIG. 3 is a cross-sectional view of a portion of the related art IPS mode LCD device. In FIG. 3, the IPS mode LCD device 95 includes a first substrate 40, a second substrate 70 facing the first substrate 40 and a liquid crystal layer 90 therebetween.
On the first substrate 40, a gate line (not shown), a common line (not shown), a data line 50, a thin film transistor (TFT, not shown), a common electrode 62, and a pixel electrode 64 are disposed. The common line is parallel to and spaced apart from the common line. A gate insulating layer 48 is formed on the gate line and the common line. The data line 50 is formed on the gate insulating layer 48 and crosses the gate line to define a pixel region P. The TFT is connected to the gate line and the data line 50 and includes a gate electrode, a semiconductor layer, a source electrode and a drain electrode.
A passivation layer 60 is formed on the TFT and the data line 50 and includes a drain contact hole (not shown) exposing the drain electrode of the TFT. The pixel electrode 64 is formed on the passivation layer 60 and connected to the drain electrode through the drain contact hole. The common electrode 62 is formed on the passivation layer 60 and connected to the common line. The common and pixel electrodes 62 and 64 have a bar shape and are alternately arranged with each other. The common and pixel electrodes 62 and 64 have a single layer of a transparent conductive material or an opaque metallic material.
On the second substrate 70, a black matrix 73, a color filter layer 76 and an overcoat layer 78 are formed. The black matrix 73 has a lattice shape to expose the pixel region P. The color filter layer 76 includes red, green and blue color filter patterns 76a, 76b and 76c corresponding to the pixel region P. The overcoat layer 78 covers the color filter layer 76. Although not shown, a spacer for maintaining a cell gap may be formed on the overcoat layer 78.
When the related art IPS mode LCD device 95 is exposed to ambient light, the common and pixel electrodes 62 and 64 have a reflectance above 60% such that there are rainbow stains and ambient contrast ratio is decreased. As a result, a displaying image quality is degraded.
In addition, the black matrix 73 has a relatively large area considering an align margin of the first and second substrates 40 and 70, an aperture ration is reduced.